preterito vs imperfecto

Basic Rules for distinguishing Preterite Perfect Indicative & Preterite Imperfect Indicative
Aspect of Verb: >>> [click for explanation of "aspect of verb"]	Perfective Verb (brief action, complete action)	Imperfective Verb (state, mental process)
Preterite Tense	1) One time, completed action. Specified number of repetitions. A single historical action.	2) Beginning or end of ongoing action. Not any more, or the implied action is done. Monumental or "photographic" perspective of an historical period. With "por", a specific limited time period. A single reaction to or perception of a specific thing/event
Imperfect Tense	3) Unspecified number of repetitions. Habitual, repeated action Used to... or Would... (past, not hypothetical/conditional)	4) Narrative Background, description of characteristics. was/were -----ing (progressive) weather as narrative background, time, age Thought processes. Feelings, attitudes

Examples of each case above
Aspect of Verb: >>>	Perfective Verb (brief action, complete action)	Imperfective Verb (state, mental process)
Preterite Tense	1) Mi abuelo se murió hace cinco años. Golpeó tres veces. Cesar cruzó el Rúbicon.	2) Lo conocí hace cinco años. Supe que el profesor es peculiar. Mi abuela fue doctora por 24 años. Lincoln vivió en el siglo XIX. Cuando llegué, el no estuvo en casa. Sentí miedo cuando vi el extraterrestre.
Imperfect Tense	3) El borracho eructaba grotescamente. De niño, iba a la playa todos los días. En la escuela secundaria, jugaba al ajedrez.	4) Era una noche oscura. Llovía fuertemente. Eran las once y media. La mujer que se sentaba en el sillón era vieja y elegante. Tenía el pelo gris y llevaba un vestido azul. Ella tenía 75 años. Ella estaba calma y seria porque quería impresionar a su familia. Sus hijos planeaban ponerla en el manicomio, pero ella iba a resistir.

A little problem with nomenclature...

While these two past tenses are usually known as the Preterite and Imperfect, they are both properly known as Preterite (which simply means past), and the significant distinction lies in the fact that the "preterite tense" is a "perfect" past while the "imperfect tense" is an "imperfect past.

Perfect = a completed action. Analogy: photograph, monument--the completion of the action is emphasized.

Imperfect = an ongoing or incomplete action (or indefinitely repeated). Analogy: a movie (in motion)--the progressive or continual nature of the action is emphasized rather than the beginning or end.

Indicative = Tense, it indicates a point in time. (As opposed to the three moods: Indicative, Subjunctive, Imperative)

Verbal Aspect: A verb can represent, without being conjugated, a tendency toward respresenting an action that happens momentarily or a process, an on-going state of being or mind.

Perfective verbs: saltar, morir, estallar, tirar, eructar

Imperfective verbs: ser, estar, querer, saber, conocer

Some verbs can shift between these aspects, in which case they correspond to boxes 1 or 4 above. They can be seen as completed actions, or as going on over time: Leer, ir, correr, comer, etc. Consequently, it is not very valuable to try to group verbs into these categories, but rather to think about what is happening in each of the four specific situations.

Most errors occur because of reading more into the sentence than is actually stated.

Verbs that Change Meaning

In the case of IR, the imperfect "iba" must be used in order to create the future modal in the past:

I was going to study, but I didn't have time. = Iba a estudiar, pero no tuve tiempo.

The preterite "fui" would refer to movement:

I went to study. = Fui a estudiar.

Saber in the preterite = found out (began to know)

Saber in the imperfect = knew

Conocer in the preterite = met (for the first time)

Conocer in the imperfect = knew

Querer in the preterite = tried, negative is refused (Quise estudiar, pero no pude. El no quiso ir.)

Basic Rules for distinguishing Preterite Perfect Indicative & Preterite Imperfect Indicative

Examples of each case above